A number of different driving methods for TFT liquid crystal panels are already known. For example, as stated in "Driver LSI Problems Solved by low Voltage Single Power Supply", Flat Panel Display 1991 (Nov. 26, 1990, Nikkei Business Publications, Inc., p. 168 to p. 172), TFT liquid crystal panel drivers (liquid crystal driving devices) can be broadly divided into two types: digital and analog. The typical structure of a conventional analog line sequential driver is shown in FIG. 38. This conventional driver contains shift register 2000, level shifter 2002, switches (analog switches) 2004 to 2018, sampling capacitors 2020 to 2026, hold capacitors 2028 to 2034, and analog buffers 2036 to 2042. Shift register 2000 shifts in synchronization with the shift clock, the output is input into level shifter 2002, and the voltage is shifted. Switches 2004 to 2010 are sequentially turned off (opened) based on the output of level shifter 2002, resulting in the sequential sampling of video signals by capacitors 2020 to 2026. When video signal sampling is finished, the output enable signal becomes valid and switches 2012 to 2018 simultaneously turn on (close). When this happens, the sampled voltages are held by capacitors 2028 to 2034 through capacitive coupling between capacitors. The voltage that is held is then buffered by analog buffers 2036 to 2042 and is output to the signal lines of the liquid crystal panel as display signals. Analog buffers 2036 to 2042 are constructed, for example, by connecting operational amplifiers to voltage followers.
The configuration of the pixel region of the liquid crystal panel is shown in FIG. 39. Signal line 2050 is connected to the source region of TFT (Thin Film Transistor) 2054, scan line 2052 is connected to the gate electrode of TFT 2054, and pixel electrode 2054 is connected to the drain region of TFT 2054. When TFT 2054 is selected by scan line 2052, the voltage difference between the voltage applied to pixel electrode 2056 and the counter voltage (common voltage) applied to the counter electrode is supplied to liquid crystal element 2058, thereby driving liquid crystal element 2058.
Liquid crystal elements degrade when direct current voltage is applied to them for extended periods. This property makes necessary a driving means in which the polarity of the voltage applied to the liquid crystal elements is inverted after a specified period of time. As shown in FIG. 40A to FIG. 40D, such known driving methods include frame inversion driving (hereafter referred to as "1V inversion driving" for the sake of convenience), scan line inversion driving (hereafter referred to as "1H inversion driving" for the sake of convenience), signal line inversion driving (hereafter referred to as "1S inversion driving" for the sake of convenience), and dot inversion driving (hereafter referred to as "1H+1S inversion driving" for the sake of convenience).
In 1V inversion driving, as shown in FIG. 40A, the polarity of the applied voltage in all pixels is the same within a single vertical scanning period (1 field, 1 frame); and the polarity of all pixels is inverted after each vertical scanning period. While 1V inversion driving has the advantage of having driver circuits that are simple and easy to control and, moreover, does not suffer from line nonuniformity, this driving means does suffer from extremely conspicuous screen flicker.
In 1H inversion driving, as shown in FIG. 40B, the polarity of the applied voltage differs for each scan line; and, under these conditions, polarity is inverted after each vertical scanning period. The advantage of 1H inversion driving is that flicker is not conspicuous and cross-talk in the vertical direction is inhibited. Conversely, however, it suffers from the drawbacks of susceptibility to horizontal cross-talk and visible horizontal stripes in video displays. This driving method is particularly effective when employing non-linear active elements (such as polycrystalline TFTs and MIMs, for example) with large off leakage currents. Large liquid crystal panels, however, suffer from a brightness gradient problem caused by parasitic resistance of the interconnect electrodes. The brightness gradient problem cannot be solved by means of 1H inversion driving.
In 1S inversion driving, as shown in FIG. 40C, the polarity of the applied voltage differs for each signal line; and, under these conditions, polarity is inverted after each vertical scanning period. The advantage of 1S inversion driving is that flicker is not conspicuous and cross-talk in the horizontal direction is inhibited. Conversely, however, it suffers from the drawbacks of susceptibility to vertical cross-talk and visible vertical stripes in video displays. Although it is possible solve the brightness gradient problem mentioned above, using elements which have large off leakage currents leads to undesirable effects.
In 1H+1S inversion driving, the polarity of the applied voltage differs for each pixel; and, under these conditions, polarity is inverted after each vertical scanning period. 1H+1S inversion driving is disclosed in, for example, "A 13-inch EWS High-Definition TFT Liquid Crystal Panel With Improved Picture Quality by Means of Dot Inversion Driving", Flat Panel Display 1993 (Dec. 10, 1992, Nikkei Business Publications, Inc., pages 120 to 123). This method has the advantages of both 1H inversion driving and 1S inversion driving; it also has the drawbacks of both. Further, the realization of this method means that the configuration and control of the driver circuits become extremely complex, thus creating the disadvantages of longer design times and higher device costs.
As described above, each of the said four driving methods has both advantages and disadvantages. Hence, the question of which of these four driving methods to use is determined by considering such things as the type and performance of non-linear active elements, the size of the liquid crystal panel, the targeted display quality, the cost of the device, and a variety of other design conditions. However, these design conditions are sometimes changed in the development process; and a change in any of the said design conditions after one of the said four methods has already been adopted will also necessitate a change in the driving method, a matter that requires tremendous labor for circuit changes and such. Therefore, a liquid crystal driver that can easily accommodate these types of design changes is desirable.
If a liquid crystal driver is to be supplied as a standard device, it should have a high degree of general versatility so that it may accommodate all users. Users of liquid crystal drivers, however, employ a variety of driving methods, such as those above. In addition to the variety of driving methods, moreover, is a wide variety of performance (operating speed, number of signal lines, etc.) requirements for liquid crystal drivers. Consequently, it has been difficult to supply a highly versatile, standard liquid crystal driver capable of answering the demands of all users. Yet this problem could also be solved if one were able to offer a liquid crystal driver that realizes all four of the said driving methods on one device without unduly enlarging the circuit.
In addition, analog buffers 2036 to 2042 (see FIG. 38), which are used in the liquid crystal driver, need to have a wide output voltage range (operating range). This is because a wide output voltage range facilitates the making of a liquid crystal panel capable of displaying multiple gray-scale levels. To obtain a wide output voltage range, it is necessary to widen the range of the supply voltage that is supplied to the analog buffers. However, to achieve this, a manufacturing process whose breakdown voltage is high must be used, which leads to the problems of increased circuit size and higher costs. For example, Japanese Unexamined Patent Application Heisei 6-222741 discloses technology of the prior art which generates a high quality display in multiple gray-scales levels using low voltage drivers. In the technology of the prior art, however, liquid crystal drivers and other peripheral circuits are not integrated on the liquid crystal panel, and analog buffers are comprised not of TFTs but of single crystal CMOS transistors. In addition, the characteristics of analog buffers comprised of TFTs, and those of analog buffers comprised of single crystal CMOS transistors differ in various respects, including such things as the width of the linear region in input-output characteristics, allowable supply voltage ranges, and offset values. Therefore, even if the said technology of the prior art were applied to an analog buffer comprised of TFTs, a high quality display having multiple gray-scales could not be obtained. In addition, there has been absolutely no disclosure in the said technology of the prior art regarding the idea for a liquid crystal driver capable of using the four driving methods together; and, moreover, the said technology of the prior art is related to digital liquid crystal drivers, not to analog line sequential drivers.
In addition, analog buffers contained in liquid crystal drivers are provided for each individual signal line of the liquid crystal panel, making the number of buffers extremely large. For example, a 480.times.640 dot full-color liquid crystal panel requires a minimum of 640.times.3 analog buffers. Also, since analog buffers pass electric current from integrated constant current supplies, there is the additional problem of finding a way to hold the current consumption of the analog buffers at a low level in order to reduce the power consumption of the overall device.
The present invention was designed to resolve the problems described above, and it is aimed at the realization of multiple driving methods which can invert the polarity of voltage applied to liquid crystal elements without unduly increasing the size of circuits in the liquid crystal driving device.
Another of the aims for the present invention is the realization of an analog buffer which is comprised of TFTs and which can switch between positive polarity and negative polarity by means of a shift in the supply voltage.
Yet another of the aims for the present invention is to hold the current consumption of the analog buffers to a low level and achieve low power consumption.